1. Field of the Invention
This invention relates to methods and apparatus for forming a metallic tube and, more particularly, to methods and apparatus for forming a metallic tube from an unpierced metallic billet.
2. Description of the Prior Art
In the art of forming metallic tubes, it is known to employ a pierced metallic billet, i.e., a longitudinally extending metallic billet which is initially drilled to provide an aperture extending along its longitudinal axis. The pierced billet is extruded longitudinally through an aperture in a die, with a mandrel positioned within the hollow center of the billet and extending along the axis of the billet into the die aperture from a location upstream of the die aperture. Subsequent drawing operations may then be utilized to reduce the tubing to a desired finished size. While such a technique is well adapted to form tubing of good quality, the initial drilling operation is generally quite costly, in terms of both time and material loss, with any subsequent drawing operations involving additional costs. Moreover, the drilling operation can be performed accurately only on relatively short billets, eliminating any possibility for the use of pierced billets in continuous extrusion machines.
It is also known to form tubes of relatively soft metallic materials, e.g., aluminum, by extruding unpierced billets of such materials through "spider dies", i.e., dies which have core mandrels attached to the die sides by means of struts. The use of "spider spider dies" with harder metallic materials is not practicable, due to the presence of very significant stresses and wear forces on the die struts as the billet material parts into several streams in advancing "spider die", struts, prior to rewelding downstream. "C"-shaped "C"-shaped "C"-shaped "C"-shaped "C"-shaped
A technique of extruding an unpierced metallic billet so as to form tubing of circular cross-section, which technique avoids the use of a spider die, is found in U.S. Pat. No. 1,847,365 to C. D. Skinner. The Skinner patent discloses three stages of billet deformation. In a first stage, a heated metallic billet is forced longitudinally through a generally C-shaped passageway, thereby imparting a generally C-shaped configuration to the cross-section of the billet in a plane perpendicular to the longitudinal direction of extrusion. In a second stage of deformation, the billet material traverses a relatively large diameter, annular zone, i.e., an annular zone having an outer diameter equal to that of the generally C-shaped passageway of the first stage of deformation, until an abrupt contact with a radially extending wall which includes a relatively small diameter, annular die aperture, i.e., an annular die aperture with an outer diameter which is substantially smaller than the outer diameter of the generally C-shaped passageway of the first stage of deformation. A terminal member projects downstream through both the annular zone and the annular die aperture from the upstream region of the generally C-shaped passageway, providing inner boundaries to the annular zone and annular die aperture. During the second stage of deformation, the billet material is so reshaped as to produce a continuously annular body of metal, with a relatively large outer diameter, equal to that of the generally "C"-shaped cross-section formed during the second stage of deformation, extending for some distance upstream of the annular die aperture. Then, in a third stage of deformation, the annular body of metal is extruded longitudinally through the annular die aperture, forming a length of tubing with a relatively small outer diameter, equal to that of the annular die aperture.
The Skinner patent does not identify the material of the billet, other than specifying that it is a metal. The billet material is apparently relatively soft, however, due to its composition and/or to the use of a heated extrusion cylinder into which the billet is placed prior to the start of the first stage of deformation. The softness of the billet material may be appreciated by virtue of the presence of the previously mentioned, abrupt contacting of the billet material with the radially extending wall which includes the relatively annular die aperture, whereupon the material of the billet first fills up a substantial volume within the annular zone adjacent to the wall, attaining an annular shape of relatively large diameter, and is thereafter extruded into tubing of much smaller diameter in a single stage of extrusion.
A technique generally similar to that of the Skinner patent is demonstrated in U.S. Pat. No. 2,698,684 to K. B. Guiney and C. R. Anderson. The Guiney et al. patent discloses a process for forming tubing of various non-circular, cross-sectional configurations, which appears to involve essentially the three stages of deformation of the Skinner patent, modified somewhat for the formation of non-circular shapes. The Guiney et al. technique is disclosed as having been performed successfully only on aluminum and its alloys, although the patent indicates a belief that this technique may have wider applicability.
While the use of "spider dies" and the techniques of the aforementioned Skinner and Guiney et al. patents may be effective to produce good tubing from unpierced billets composed of relatively soft or heat-softened metals, such as aluminum, it would clearly be advantageous to provide methods and apparatus for forming high quality tubing, preferably continuously, from an unpierced billet composed of any relatively soft or relatively hard metal, while avoiding any necessity for heating the billet prior to the forming operation.